1. Field of the Invention
The invention relates to an EGR (exhaust gas recirculation) system for an internal combustion engine, and a method for controlling the same.
2. Description of the Related Art
EGR, i.e., a technology for providing communication between an exhaust passage and an intake passage and recirculating a portion of the exhaust gas back to an internal combustion engine has been suggested in order to reduce the amount of nitrogen oxide (NOx) generated in the process of burning fuel in the internal combustion engine.
A technology that makes it possible to perform EGR in a broader operating range of an internal combustion engine has been suggested in, for example, Japanese Patent Application Publication No. 2004-150319 (JP-A-2004-150319). JP-A-2004-150319 describes a system that includes a high-pressure EGR unit and a low-pressure EGR unit, and that performs EGR while changing the EGR unit used for EGR between the high-pressure EGR unit and the low-pressure EGR unit or using both the high-pressure EGR unit and the low-pressure EGR unit in combination, based on the operation mode of an internal combustion engine. The high-pressure EGR unit recirculates a portion of the exhaust gas back to the internal combustion engine through a high-pressure EGR passage that provides communication between an exhaust passage, at a portion upstream of a turbine of a turbocharger, and an intake passage, at a portion downstream of a compressor of the turbocharger. The low-pressure EGR unit recirculates a portion of the exhaust gas back to the internal combustion engine through a low-pressure EGR passage that provides communication between the exhaust passage, at a portion downstream of the turbine, and the intake passage, at a portion upstream of the compressor.
With this EGR system, the amount of exhaust gas, which is recirculated back to the internal combustion engine by the high-pressure EGR unit (hereinafter, referred to as the “high-pressure EGR gas”), and the amount of exhaust gas, which is recirculated back to the internal combustion engine by the low-pressure EGR unit (hereinafter, referred to as the “low-pressure EGR gas”), are adjusted to the specified values determined in advance. When the amount of high-pressure EGR gas and the amount of low-pressure EGR gas are at the specified values, optimum predetermined engine characteristics such as the exhaust gas properties and the fuel efficiency characteristics are achieved.
During the steady operation of the internal combustion engine, the high-pressure EGR gas amount and the low-pressure EGR gas amount are appropriately adjusted to the specified values. However, in the transitional operation period in which the operation mode of the internal combustion engine is changing, especially the amount of low-pressure EGR gas, which flows through a flow path that is relatively long, is likely to deviate from the specified value. This may cause inconveniences such as generation of smoke.